1. Field of the Invention
This invention relates a pattern evaluation method for defective parts or corrected parts of a photomask, and more particularly to a pattern evaluation method using an image quality evaluation apparatus which has the same-wavelength light source and the same optical system as those of an exposure device used for wafer transfer.
2. Description of the Related Art
One known photomask pattern evaluation method is to evaluate the effect of an abnormal part including a defective pattern of a photomask in wafer transfer. Such a pattern evaluation method is used mainly in the process of correcting a defective part of a photomask. In the method, an image quality evaluation apparatus is used. The image quality evaluation apparatus, which has the same-wavelength light source and the same optical system as those of an exposure device, images and analyzes a wafer transfer image as an Aerial Image Measurement System (AIMS), produced by Carl Zeiss, Inc., does.
To evaluate a pattern, the image of an abnormal part, such as a defect in a wafer transfer pattern image or a defect-corrected part is imaged with an image quality evaluation apparatus. On the basis of the image of an abnormal part and the image of a normal part corresponding to the abnormal part (the part obtaining by removing the defects from the abnormal part, or another part whose pattern layout approximates to the part obtained by removing the defects from the abnormal part), a light intensity distribution of the abnormal part is compared with that of the normal part, thereby calculating the variation in, for example, the transmittance of the abnormal part and the dimensions to be transferred to the wafer with respect to the normal part. Then, the pattern is evaluated by determining whether the variations are within an allowable range in a wafer lithographic process.
In such an evaluation method, it is desirable that the image obtained by the image quality evaluation apparatus should coincide completely with a spatial image actually reaching the wafer surface. For various reasons, however, the former actually differs slightly from the latter. One great factor for this is the aberration of the optical system of the apparatus. When the pattern of an abnormal part is evaluated with the image quality evaluation apparatus, measurements are made automatically, including errors caused by the aberration. Since the aberration of the quality evaluation apparatus acting as an inspection apparatus is relatively small, the aberration has caused little trouble.
However, as patterns have been miniaturized further, the effect of aberration has increased, which makes pattern evaluation more difficult. To reduce the aberration, the body of the apparatus has to be improved. The only way to achieve this is either to increase the performance of each of the lenses or to adjust the aberration correction mechanism in a suitable way. However, the aberration cannot be made so small that its effect can be ignored. Depending on the photomask pattern, the effect of the aberration can appear significantly.
Dominant aberrations in evaluating patterns with the image quality evaluation apparatus include coma aberration and astigmatism. Astigmatism causes a phenomenon where the horizontal light condensing position of the optical system differs from the vertical one, resulting in a decrease in the measurement accuracy of the mask pattern. It is said that coma aberration is observed as a result of the disruption of the balance of the sub-peak at the mask pattern edge part. The disruption of the balance is caused by a phenomenon where light does not converge at a point on an imaging surface and forms a fan-like image in such a manner that light leaves traces. The effect of the aberrations makes the light intensity distribution asymmetrical in a part where, for example, a densely arranged pattern part is close to a roughly arranged pattern part, which makes pattern evaluation difficult.